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Illusion makes people ‘feel’ force field around their body

There’s something in the air

Dirk Fellenberg/Plainpicture

By Anil Ananthaswamy

OUR brains are aware not just of our bodies but also the immediate space around us. Now, a twist on the classic rubber hand illusion has let people “feel” this space – a sensation they liken to perceiving a “force field”.

Neuroscientists have known for decades that our brains contain representations of the area surrounding us, known as peripersonal space. This allows us to grasp objects within our reach and helps to protect us.

For example, imagine you are walking through the woods, when a low-hanging branch suddenly appears in your peripheral vision. You’ll instinctively duck to dodge it: your sense of peripersonal space has helped you avoid being hit.

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Hard neuroscientific evidence on the phenomenon appeared in the late 1990s in animal studies. Michael Graziano at Princeton University and his colleagues found that some neurons in monkey brains fired not only when an object touched the body, but also when the object came near it. Upon stimulating these neurons, they found that the monkeys would reflexively move their heads and limbs as if defending themselves – for example, grimacing and closing their eyes.

Although no one has repeated the experiments in humans, there is evidence that certain regions of our brain deal specifically with peripersonal space. For instance, some people who have strokes in the right posterior parietal lobe cannot sense peripersonal stimuli on the left side of their body, but can perceive things further away on that side in the normal way.

“This suggests that there is a representation similar to those found in monkeys in the human brain,” says Arvid Guterstam of the Karolinska Institute in Stockholm, Sweden. Now, Guterstam and his colleagues have tricked humans into feeling our peripersonal space.

They turned to the classic rubber hand illusion, in which a paintbrush is used to stroke a volunteer’s hand hidden from view and an adjacent, visible rubber hand, at the same time, speed and place on both. Within minutes, most people report feeling the brush on the rubber hand as if it belonged to them.

In the new study, which involved 101 adults, the researchers never brushed the rubber hand directly. Instead, they moved the brush above it, again at the same time as brushstrokes that touched the real hand. Most volunteers reported feeling a “magnetic force” or “force field” between the paintbrush and the rubber hand below – as if the brush was encountering an invisible barrier. The volunteers also felt a sense of ownership of the fake hand (Cognition, doi.org/bkc9).

For decades, neuroscience has filled in our knowledge of the special buffer zone around the body, says Graziano. “Now we have a clever way to get at the phenomenon.”

This article appeared in print under the headline “We can sense our invisible force field”